Field of the Invention
The present invention relates to an image pickup apparatus, such as a digital camera, a method of controlling the same, and a storage medium, and more particularly to an image pickup apparatus that is capable of detecting flicker and reducing the influence of the flicker when photographing is performed under an artificial light source, such as fluorescent lighting.
Description of the Related Art
In general, in an image pickup apparatus, such as a digital camera, an image signal obtained by a solid-state image pickup device, such as a CCD image sensor or a CMOS image sensor, is converted to a digital signal by analog-to-digital conversion, and is recorded as image data in a storage medium. Some of such image pickup apparatuses include a moving image photographing function in addition to a still image photographing function.
Incidentally, when moving images are photographed by an image pickup apparatus of the above-mentioned type under fluorescent lighting driven for lighting by a commercial power supply, brightness sometimes varies from frame to frame. Further, it is known that horizontal light and dark stripes are formed in the same frame.
Such light and dark variation is called flicker. Flicker caused by a commercial power supply frequency of 50 Hz can be reduced by controlling an electronic shutter at a shutter speed of n/100 sec (n is a natural number). Further, flicker caused by a commercial power supply frequency of 60 Hz can be reduced by controlling the electronic shutter at a shutter speed of n/120 sec.
To calculate a shutter speed (exposure time) for reducing flicker, it is necessary to calculate a period of flicker. For example, there has been proposed a method in which flicker is detected by integrating an image signal obtained by the CMOS image sensor over a time period not shorter than at least one horizontal period, and comparing integrated values of each two successive frames (see Japanese Patent Laid-Open Publication No. 2007-60585).
Further, when a still image is photographed under a flicker light source, if the photographing is performed with a high-speed shutter, variation in the exposure and color temperature of the still image is sometimes caused from one frame to another due to the influence of flicker. Further, also in the case of still image photographing, unevenness of exposure and unevenness of color are sometimes caused in one frame.
To cope with such exposure unevenness and color unevenness, there has been proposed e.g. an image pickup apparatus which is configured to reduce flicker by detecting the period and phase of flicker, and performing exposure in synchronism with a peak of the light amount of the flicker, at which the light and dark variation is small (see Japanese Patent Laid-Open Publication No. 2004-193922).
On the other hand, when reading control of an image pickup device is performed in order to detect flicker, a time delay is caused in the image pickup apparatus before the start of photographing. To solve this problem, there has been proposed an image pickup apparatus which includes a plurality of image pickup units (e.g. image pickup devices) and is configured such that flicker correction information is obtained for storage according to an image signal which is obtained by one of the image pickup units when the image pickup apparatus is operated (see Japanese Patent Laid-Open Publication No. 2007-243833).
In the image pickup apparatus disclosed in Japanese Patent Laid-Open Publication No. 2007-243833, when the image pickup unit is switched to another unit or when photographing is started next time, the stored flicker correction information is used, whereby photographing is immediately started and correction of flicker is performed without performing a flicker detection process again.
In the image pickup apparatus disclosed in Japanese Patent Laid-Open Publication No. 2007-243833, insofar as flicker correction information can be commonly used between the image pickup units, it is possible to perform correction of flicker. However, if it is impossible to commonly use flicker correction information between the image pickup units, it is required to acquire flicker correction information at least in one image pickup unit, again, which causes time delay before the start of photographing.